Conventionally, in a printing apparatus such as a barcode printer and the like, printing is carried out on a sheet-like image receiving medium (hereinafter referred to as a “sheet”) such as paper, and then the printed sheet is issued. The sheet to be subjected to printing processing by the printing apparatus is stored in a paper tray in the printing apparatus. Conventionally, a PE sensor (transmission sensor) and a plastic plate are arranged, and the existence of detection of the light emitted from the PE sensor is used to determine the existence of the sheet stored on the paper tray.
However, in a conventional printing apparatus, the degree of the residual quantity of the sheets cannot be detected. Thus, sheets are not replenished until the sheets stored on the paper tray are used up; as a result, downtime occurs frequently. Such a problem occurs not only in the printing apparatus but also in other apparatuses in which sheets are stored in a paper tray.